The present invention involves an improvement to the feed and product in a naphtha reforming process. In particular the present invention provides an adsorbent that is effective for trace sulfur removal for feeds to naphtha reforming units as well as product streams from such units.
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased “octane,” or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Growing demand for high-purity aromatics as petrochemical intermediates also is a driving force for the upgrading of naphtha.
Catalytic reforming is a major focus, as this process generally supplies 30-40% or more of the gasoline pool and is the principal source of benzene, toluene and xylenes for chemical syntheses. Increased reforming severity often is accompanied by a reduction in reforming pressure in order to maintain yield of gasoline-range product from the reforming unit. Both higher severity and lower pressure promote the formation of olefins in reforming, and the 1-2+% of olefins in modern reformats contribute to undesirable gum and high endpoint in gasoline product and to particularly troublesome impurities in recovered high-purity aromatics streams.
Catalytic reforming catalysts are sensitive to sulfur compounds that may be present in the feedstock at levels of about 10 parts per million (ppm). Optimally, it is desired to reduce the level of sulfur compound contamination to levels of about 1 to 0.1 ppm.
Guard beds with supported copper oxide (CuO) have been used for feed purification in catalytic reforming units. Unfortunately, the CuO reduces in the, at the typical operating temperatures for the liquids being treated. Typically in prior art systems, the reduction of CuO occurs rapidly, and large amounts of water are produced. The excessive moisture is disadvantageous to the operation of the catalytic reforming catalyst, causing undesirable side reactions. In addition, there is the undesired exotherm.
Copper containing materials are widely used in industry as catalysts and sorbents. The water shift reaction in which carbon monoxide is reacted in presence of steam to make carbon dioxide and hydrogen as well as the synthesis of methanol and higher alcohols are among the most practiced catalytic processes nowadays. Both processes employ copper oxide based mixed oxide catalysts.
Copper-containing sorbents play a major role in the removal of contaminants, such as sulfur compounds and metal hydrides, from gas and liquid streams. One new use for such sorbents involves the on-board reforming of gasoline to produce hydrogen for polymer electrolyte fuel cells (PEFC). The hydrogen feed to a PEFC must be purified to less than 50 parts per billion parts volume of hydrogen sulfide due to the deleterious effects to the fuel cell of exposure to sulfur compounds.
Copper oxide (CuO) normally is subject to reduction reactions upon being heated but it also can be reduced even at ambient temperatures in ultraviolet light or in the presence of photochemically generated atomic hydrogen.
The use of CuO on a support that can be reduced at relatively low temperatures is considered to be an asset for some applications where it is important to preserve high dispersion of the copper metal. According to U.S. Pat. No. 4,863,894, highly dispersed copper metal particles are produced when co-precipitated copper-zinc-aluminum basic carbonates are reduced with molecular hydrogen without preliminary heating of the carbonates to temperatures above 200° C. to produce the mixed oxides. However, easily reducible CuO is disadvantageous in some important applications, such as the removal of hydrogen sulfide from gas and liquid streams when very low residual concentration of H2S in the product is required
The residual H2S concentration in the product gas is much higher (which is undesirable) when the CuO reduces to Cu metal in the course of the process since reaction (1) is less favored than the CuO sulfidation to CuS.2Cu+H2S=Cu2S+H2  (1)The known approaches to reduce the reducibility of the supported CuO materials are based on combinations with other metal oxides such as Cr2O3. The disadvantages of the approach of using several metal oxides are that it complicates the manufacturing of the sorbent because of the need of additional components, production steps and high temperature to prepare the mixed oxides phase. As a result, the surface area and dispersion of the active component strongly diminish, which leads to performance loss. Moreover, the admixed oxides are more expensive than the basic CuO component which leads to an increase in the sorbent's overall production cost.
The present invention comprises a new method to improve feed purification in a catalytic naphtha reforming process by using a supported CuO adsorbent which contains chloride as a means to decrease the tendency of CuO to be reduced to low valent state, especially Cu metal. Surprisingly, it has now been found that introducing chloride either in the basic copper carbonate, which serves as CuO precursor, or into the intermediate CuO-alumina adsorbent leads to material having improved resistance to reduction in catalytic reforming processes.